SATELLITE-TRACKING PROGRAM — ^HAYES 347 



crease and therefore its density at a given altitude. It must be added 

 that scientists do not yet understand precisely how this heating occurs. 



The next discovery by Dr. Jacchia was that the atmosphere at a 

 given height is denser in the illuminated — that is, the bright — hemi- 

 sphere than it is in the night hemisphere. In other words, the atmos- 

 phere bulges out toward the sun. This diurnal bulge is another phe- 

 nomenon caused primarily by the extreme ultraviolet radiation from 

 the sun. 



At a height of 150 km., surfaces of equal density in the atmosphere 

 are nearly concentric with the earth. At higher altitudes, however, 

 a slight bulging out occurs around the point that is at the same latitude 

 as the subsolar point but shifted 2 hours in longitude. This bulging 

 out reaches a maximum in the region between 600 and 1,000 km. ; the 

 bulge then decreases in the helium and hydrogen regions of the atmos- 

 phere. The temperature goes up much more sharply in the bulge. 

 At the height of Vanguard I, for example, the density of the atmos- 

 phere in 1958-59 varied by nearly one order of magnitude across the 

 bulge ; the density increased by nearly one order of magnitude going 

 through its center, and then decreased. 



A fourth effect of solar radiation is the semiannual variation. In 

 1960 Professor H. K. Paetzold found from Dr. Jacchia's observations 

 of Vanguard I and Satellite 1958 Alpha that there are indications of a 

 small semiannual oscillation in the drag. His discoveiy was then sub- 

 stantiated by Priester and Jacchia. The maxima and minima of this 

 oscillation agree with the maxima and minima of the semiannual 

 oscillation in the geomagnetic indices and with the maxima and minima 

 of aurorae and magnetic disturbances. 



Again, the mechanism of this variation is not imderstood. The 

 changing dip of the magnetic axis of tlie earth with respect to the 

 "solar wind" has been invoked to explain the effect, but this explana- 

 tion seems to meet with increasing difficulties. 



From all of these observations and deductions, a new model of 

 atmospheric heating resulted. The troposphere extends to between 

 8 and 12 km. from the ground. The ground is heated by visible radia- 

 tion ; then the heat is transferred from the ground to the atmosphere by 

 conduction and convection. Above the troposphere is the ozonosphere, 

 the layer of atmosphere that contains a quantity of ozone which absorbs 

 the near ultraviolet ; most of this region is between 25 and 40 km. above 

 the earth. The layer above is heated from the ozonosphere in the same 

 way that the troposphere is heated by the ground. These facts had 

 already been available, however, to estimate the nature and extent of 

 heating in the upper atmosphere above 100 km. 



